$\begingroup$depends on what you define as ocean. after a few terapascals, water will metallicize.$\endgroup$
– Serban TanasaNov 26 '15 at 13:27

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$\begingroup$Jupiter has a planet wide liquid hydrogen ocean that is thousands of miles deep.$\endgroup$
– RBarryYoungNov 26 '15 at 16:52

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$\begingroup$What do you want to do with this "ocean?" Understanding what you think you will do with the ocean may help us understand your particular definition of "ocean." Even simple words like "ocean" get complicated with extremes like "How deep could it possibly be." Everything eventually gets complicated when you take it to an extreme.$\endgroup$
– Cort AmmonNov 26 '15 at 18:03

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$\begingroup$Does it have to be water?$\endgroup$
– SchwernNov 26 '15 at 21:40

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$\begingroup$Keep in mind that much of the gravity-based pressure exerted on the water can be counteracted by centrifugal force for a sufficiently quickly spinning planet, at least on the equator (of course, it would be a highly elliptical planet)$\endgroup$
– Fabio BeltraminiNov 27 '15 at 9:28

6 Answers
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Planets can (theoretically) be made of nothing but water. Although after a few hundred kilometers, the water in the center may be turned into some exotic version of ice due to the pressure, so one would not be able to "dive through" the planet, and thus the ocean would have a limited depth.

Ganymede, the largest of Jupiter's moons, might have an interior that's fully liquid, for example.

$\begingroup$Well, by definition "ocean" should be liquid, so you would not be able to go deeper than a few hundreds km, because water would necessarily turn into ice under the pressure.$\endgroup$
– Radovan GarabíkNov 26 '15 at 13:25

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$\begingroup$well, while it may be technically called 'Ice VII', 'Ice X' and 'Ice XI', it's an ionized and high temperature fluid really. Not your regular ice from the skating rink.$\endgroup$
– Serban TanasaNov 26 '15 at 20:59

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$\begingroup$I dunno about "hundreds of kilometers". By my calculations and this diagram, the pressure at 64.5 km would cause liquid H2O at 273.15K (0C) to phase into exotic Ice VI. If the water were at 355K (82C, and unfeasibly hot for a scuba diver) however, it would take a depth of over 200 km to reach exotic ice territory. I guesstimate that at a swimmable temperature of about 300K, ice formation happens at 100km pressure.$\endgroup$
– Iwillnotexist IdonotexistNov 26 '15 at 22:07

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$\begingroup$@IwillnotexistIdonotexist: I wonder if you could get more liquid water with a smaller planet (so less gravity and pressure)?$\endgroup$
– Dan SmolinskeNov 26 '15 at 23:52

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$\begingroup$@DanSmolinske Yes actually. I made my calculations based on an Earth-sized planet, where the mass of even 60km of water is negligible when compared to the rocky 6000-km planet radius. It would be fun to figure out the radius of a blob of water with core water pressure >650 MPa, but that gets slightly more involved than my approximations. Such a planet will have several cute properties: Big splashes on asteroid glancing blows, meteorites sink to the bottom, and it would be desirable to be as deep as possible so water can slow down inbound space junk. Justification for the scuba-diving plot!$\endgroup$
– Iwillnotexist IdonotexistNov 27 '15 at 11:50

On Earth, deep water has a temperature of around 0C. At that temperature, water has a phase transition from liquid to ice VI around 632 MPa. Under Earth-like gravity, this pressure requires a water column of about 65 kilometers; if your ocean is any deeper, the water at the bottom will solidify. Since ice VI, unlike the familiar ice Ih, is denser than water, it will stay at the bottom.

I'm ignoring the compressibility of water (which will tend to decrease the depth) and the salinity of water (which tends to both increase it by freezing-point depression and reduce it by increasing the density of water).

$\begingroup$I think this is an important point for OP as he/she wants the planet to be "really big": the smaller the planet, the deeper the ocean can be!$\endgroup$
– CephalopodNov 28 '15 at 20:38

$\begingroup$Until the planet's radius is less than the distance required to reach 632 MPa! ;) (Does this have a named radius, like the Schwarzschild Radius or Roche Limit? If not, it should! At least it should within the fiction being written because why not?)$\endgroup$
– Draco18s no longer trusts SEDec 2 '15 at 19:13

This is implicit in the other answers, but it deserves to be made explicit: if you want a deeper ocean, a smaller planet will be better than a big one.

The main thing limiting the depth of a liquid ocean is that when the pressure gets to around 1GPa, the pressure will cause the water to become ice, even at warm temperatures. This is a special forms of ice (actually one of several different special forms of ice) that is denser than water, so it sinks and forms the ocean floor. Probably most of the oceans on the icy moons in the Solar system, such as Europa, have exotic ices at the bottom.

The pressure at a given depth is given by the density of water multiplied by the height of the water column, multiplied by the strength of the planet's gravity. Thus the higher gravity of a Jupiter-like planet would mean that exotic ices would start to form at a shallower depth of water than on an Earth-like planet, whereas on a planet smaller than Earth you could have deeper water before you reach the critical pressure. The relationship is linear, so a planet with half the gravity can have oceans twice as deep.

$\begingroup$But the weight of the oceans means that you can't have half the gravity presumably?$\endgroup$
– Tim BNov 27 '15 at 16:19

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$\begingroup$@TimB, until you get extremely small, the oceans are a trivial component of the planet's total mass. If I've done the math correctly, a Mars-sized planet would be 4% ocean by mass, while a Moon-sized one would be about 25% ocean by mass.$\endgroup$
– MarkNov 27 '15 at 18:58

$\begingroup$@TimB Mark is right, the oceans will be a small part of the mass for most reasonably-sized planets. I guess when you get really small there's a point where the mass is mostly ocean. At the smallest extreme, there will be maximum size where a planet could be made entirely out of water without forming exotic ices in the middle. It would be interesting to calculate that size, but I'm fairly sure it will be so small that it couldn't hold an atmosphere (required in the question), and since low pressure causes water to boil, the whole planet would eventually evaporate into space.$\endgroup$
– NathanielNov 28 '15 at 1:09

Assuming water to be water that is less than 20% dissociated and setting the compressibility at zero we have a reasonable maximum at about 300Mbar with a temperature of ca. 1200K Redmer-Icaurus2011. Assuming $g=10m/s^2$ (close to Earth's) we can relate each 10 m height of the water-column to 1 bar pressure. 1 Mbar = 1000 bar for a water-column of 10 km height, makes 3,000km which is about the radius of Earth.

There must be a temperature of 12000 K at the center (maybe a small nuclear reactor like inside the Earth) and > 274 K at the surface if the atmosphere has a pressure of 1 bar.

This won't work exactly as described because I made a bit too many assumptions. E.g.: I ignored the gravitational pressure of the weight of the water-column completely which would add to the temperature as $(G * mass * mm)/(2 * k * r)$ with gravitational constant $G = 6.67428 * 10 ^{-11}$, Boltzman constant $k = 1.3806504 * 10 ^{-23}$, mm = molecular mass, and r = radius, which gives the temperature at the center by gravitation alone.

But it would allow for a couple of hundred kilometers of liquid water with a comfortable temperature, given a "well tuned" reactor in the middle and an atmosphere that holds some of the energy of the central star(s) back to keep the surface warm.

So: it's possible without any visible hand-waving and expensive unobtanium.

Water (or ocean) planet is a hypothetical planet composed (mostly) of water. It might have a smaller rocky core, but the decisive factor is the formation of exotic ices under pressure. The ocean depth can reach hundreds of kilometers (depends on the temperature, gravity etc.), without sharp boundary with the ice "mantle". It would necessarily have an atmosphere due to outgassing from the liquid surface, if the temperature is within liquid water range (otherwise you'd get Europa scenario).

If your planet has enough carbon dioxide, at about 72 atmospheres (assuming around 300K, the carbon dioxide will liquify forming an ocean. As you go down in the ocean, depending on the temperature gradient, it will either become solid around 6000 bar (60 Km down), but with mild warming, you might be able to go many hundreds of Km down before you hit solid CO2. (BONUS: It would be supercritical carbon dioxide which is a weird hybrid of a liquid and a gas).